1. Field of the Invention
This invention relates to the field of communications. More specifically, the invention comprises a method for programming an active HDMI cable in order to optimize its performance.
2. Description of the Related Art
The present invention is an enhancement of prior art HDMI cable technology. “HDMI” stands for “High-Definition Multimedia Interface.” HDMI cable is commonly used to carry high-definition video and audio data between television-related components. For example, HDMI cables are often used to connect satellite “set top boxes” to associated video displays and surround-sound systems. However, HDMI cables carry more than just video and audio information. While there is some variability within the defined HDMI standards, most HDMI cables carry: (1) high-definition digital video data, (2) digital audio data, (3) CEC (consumer electronics connection) command data, (4) Ethernet data, (5) digital content encryption data, and (5) component “handshake” data.
The beauty of the HDMI standard is that all these connections are made via a single integrated cable. FIG. 1 shows a representative HDMI cable 10. An HDMI cable typically includes an integrated conductor bundle 16 having a termination 18 on each end. The conductor bundle includes multiple isolated conductors within a single protective jacket. Termination 18 provides a transition for the conductor bundle to connector 12.
Connector 12 opens into a cavity including numerous pins 14. The pins actually make the electrical connection when the connector is plugged into an HDMI receptacle. Connector 12 is referred to as a “male” connector in the traditional nomenclature of electrical connections. However, the reader will note that the connector actually includes a cavity that receives a protruding part on a corresponding HDMI receptacle. The pins 14 are biased inward. They maintain a clamping force when connector 12 is plugged into an HDMI receptacle, thereby making contact with the electrical “lands” provided on the receptacle.
FIG. 2 shows a complete HDMI cable 12 in a coiled state. Conductor bundle 16 includes a termination 18 on either end. The termination provides a transition between the parallel or twisted conductors in the conductor bundle and the connector 12. Bach conductor within the bundle is electrically connected to a pin within the connector. Termination 18 is commonly molded over the completed connections between the conductors and the pins within the connector (a process known as “overmolding”). The termination is relatively rigid, in order to provide strain relief for the connections between the conductors and the pins.
Conductor bundle 16 is preferably flexible, so that the cable may be bent and routed as desired. The flexibility of prior ait HDMI cables is limited by the diameter “D” and the materials selected. HDMI cables are known to have a fairly large diameter in comparison to the cable's length. For example. HDMI cables having a length of 1 to 3 meters typically have a diameter of 7 mm (0.275 inches). This relatively large diameter makes conductor bundle 16 relatively stiff, which can interfere with the routing of the cable around corners.
FIG. 3 shows an HDMI cable connecting source 20 to display device 22. A “source” can be any type of device that transmits data for use by an HDMI cable. Examples include satellite or cable set top boxes, DVD players, audio processing units, video recorders, etc. The term “display device” refers in this example to a video display that may or may not include audio capability as well. A good example is a high-definition television. The reader should bear in mind that a “display device” is only one type of device that might use an HDMI cable. One might more generally refer to the device in the position of display device 22 as a “sink.” The term “sink” might include a surround-sound unit, a digital video recorder, or a computer. The reader should bear in mind throughout this disclosure that whenever the term “display device” is used one could substitute some other type of “sink” device.
Both source 20 and display device 22 are equipped with an HDMI receptacle 24. The connector on each end of the HDMI cable is plugged into an HDMI receptacle. Thus, the HDMI cable connects the source to the display device. Since the HDMI cable provides video data, audio data, and auxiliary conduits facilitating digital communication between the devices, it is often the only connection needed. This “one wire” approach is often touted as HDMI's main benefit.
However, existing HDMI cables have some drawbacks. First, the HDMI standard specifies a minimum mechanical extraction force of only 9.8N (2.2 pounds) for the HDMI connectors. This fact means that an HDMI connector is relatively easy to pull free of its socket. The available retention force would be sufficient if the HDMI connectors were attached to a thin and flexible cable. This is not typically the case, however. As mentioned previously, the conductor bundle portion of an HDMI cable is often relatively thick and rigid. When the cable is bent ami flexed between components the cable bundle itself creates extraction forces on the connectors. These forces may actually pull the connector free of the receptacle. Even if the connector is not pulled free, the forces placed on the connector may cause some of the individual pins within the connector to become disconnected.
The common thickness and rigidity of HDMI conductor bundles is driven by the existing HDMI technology, and it is important for the reader to understand the limitations of this technology before the present invention is discussed.
FIG. 4 shows the prior ait electrical connections that are actually made by an HDMI Type A cable. Source connector 26 receives inputs on 19 different pins (numbered 1 through 19 in the view). An electrical connection is made to corresponding pins (1-19) on sink connector 28. Pins 1-12 carry the high-definition video signals. The video signals are carried, in four “channels,” commonly referred to as the red, green, blue, and clock channels. Transition-minimized differential signaling (“TMDS”) is used for each of these channels. The “red” channel is designates as “TMDS D2” and is carried by three pins.
“Transition-minimized” (“TM”) refers to a technique of bit encoding that clusters 1's and 0's together in order to minimize 1 to 0 or 0 to 1 transitions. A digital transition creates an edge of a square wave. This edge creates unwanted harmonic energy and can create electromagnetic interference. IM encoding minimizes these transitions.
“Differential signaling” (“DS”) refers to a technique of sending two complementary signals on two paired wires. The polarity of the two (DC) signals is opposite. The two signals are typically fed into a subtracter on the receiving end. This device has the effect of doubling the amplitude of the desired signal while canceling any unwanted noise picked up by the transmitting lines. TMDS is the combination of “TM” and “DS” signaling.
The “TMDS D2” channel shown in FIG. 1 is carried by pins 1-3. Pin 1 is the positive half of the differential signal. Pin 3 is the negative half. Pin 2 is connected to a shield surrounding the positive and negative lines.
The “TMDS D1” channel carries the “green” signal. Pin 4 is the positive half of this differential signal while pin 6 carries the negative half. The “TMDS D0” channel carries the “blue” signal. Pin 7 carries the positive half of this signal while pin 9 carries the negative half. The “TMDS clock” channel carries the clock signal for the video feed on pins 10 and 12.
The reader will thereby appreciate that the HDMI standard uses three separate TMDS channels for the additive primary colors used in creating displayed video images. These signals are transmitted at a very high rate (3.4 GHz). Even a small change in the propagation characteristics of the copper conductors used to carry the components can cause problems. “Intra-pair skew” is defined as a difference in propagation speed between the positive and negative lines in a differential pair (such as the conductors connecting the two Pin 1's and the conductors connecting the two Pin 3's). A slight difference in arrival times for the two components of a pair can cause a data bit to fall outside of the decision boundary defined for that bit (referenced to the clock signal). Digital 1's can then become 0's and vice-versa. Small difference in conductor length, twists, and kinks in the cable can all cause these problems.
The remaining pins in the prior art HDMI cable connectors serve additional functions. Pin 13 carries a Consumer Electronics Connection (“CEC”) data. CEC data allows one HDMI-compatible device to control another. For example, a user can employ a remote control for a television to also control a DVD player using commands sent over the CEC.
Pins 14 and 19 in conjunction serve as an Ethernet connection. Pin 14 is the positive portion of the pair and Pin 19 is the negative portion (as well as serving an additional function).
Pins 15 and 16 are used for a Display Data Channel (“DDC”) that operates over an I2C bus. The I2C bus is in fact significant to the present invention as it is used as the carrier for signals that program the active components of the inventive HDMI cable.
Pin 17 provides a ground connection, while Pin 18 provides +5 V DC. Pin 19 serves as the negative pair for an Ethernet connection when such a connection is in use. In addition, Pin 19 provides a “hot plug detect” function. The source device monitors this pin. When a receiving device is initially plugged into the cable, the source device will detect a 5 V signal on Pin 19. This then initiates an exchange of data between the source and sink devices.
The cable connection schematically illustrated in FIG. 4 represents a purely passive device. The source and sink devices are active, but the connection between the two is simply a bundle of passive connectors. Various prior art schemes are used to facilitate accurate data transfer such as shielding, the use of differential signaling, etc. However, the use of a passive cable introduces certain well known problems.
The simplest problem is that of voltage drop. All the conductors shown in FIG. 4 have resistance. As the length of the cable increases, the voltage drop across the conductors increases. The result is the eventual failure of some of the signals. For example, the “hot plug detect” signal may fall so far below the nominal 5V that it is simply not detected. The prior art solution to this problem has been to use larger gage conductors. This approach works to a certain extent, bat h causes the cables diameters to be larger and it causes the cables to be stiffer.
A second recognized problem is that of tolerance growth. The characteristics of the prior art conductors all have associated tolerances. An example would be the conductor's resistance and impedance. The tolerances generally vary proportionally with length. As an example, if a 1 meter cable has an impedance tolerance of +/−0.1 ohms, then a 2 meter cable constructed the same way would typically have an impedance of +/−0.2 ohms. This is inherent in a passive conductor. Because the tolerances grow with increasing cable length, a point will be reached where the conductors are out of tolerance (especially critical with differential pairs). This issue limits the length of cable.
A third recognized problem is that of rise time, particularly on the I2C data bus. This bus is a simple serial interface that transmits data at a rate of 100 KHz. Rise time is driven in part by the capacitance and inductance of the receiving device. Data is bidirectional on this bus so the receiving device could be either the source or the sink. However, rise time problems resulting from the sink characteristics are most common. Increasing cable length tends to increase rise time, and this factor has also limited the effective length of prior art HDMI cables.
All these issues are inherent in the prior art HDMI cable design. The solution has been to (1) limit cable length, and (2) use large conductors. Limiting the cable length has obvious disadvantages. The use of large conductors has made the HDMI conductor bundles large in diameter and, as a result, fairly stiff. It would be advantageous to provide a solution that provides a longer cable length while retaining a relatively small diameter cable. The present invention provides such a solution.